This invention relates to optical scanning carriages and particularly to controlling the movement of such a carriage with the use of dashpots. The invention has particular application for use with optical scanning carriages in automatic photocopying machines.
With the the continued development of automatic photocopying machines the desire for higher speed and more compact machines, optical scanning systems with folded optical path such as are used in the Xerox 3100 and 3300 products have been employed. In these systems a document is placed on a platen and optically scanned by a moving carriage. For each copy desired the original document is scanned which means that the processing of successive copies must wait until the scanning mechanism is returned to the start of scan position following scanning the original document for the preceding copy. If the rate of return of the scanning mechanism is the same as the scanning rate the machine will be operating in a non-productive mode one half the time. To reduce this period of non-productive time it has been common to speed up the rate of return of the scanning carriages on the rescan cycle. However, the scanning carriages in these photocopiers carry lamps to illuminate a document while it is being scanned and mirrors to reflect the illuminated images to the imaging member. These lamps, especially when they are hot, are delicate and sensitive to accelerations and may be easily broken. The mirrors if handled too roughly have a tendency to crack but more importantly may be jarred from the precise alignment required for faithful reproduction of the original document. Typically these carriers are driven in one direction by means of cables connected to the main drive through a clutch and returned at much higher speed under the influence of a spring when the clutch is disengaged. At the end of the return scan the carriage collides with a buffer of some type to stop the carriage. While various means have been used to soften the collison and thereby preserve lamps and mirrors, the use of a fixed pneumatic dashpot positioned at the end of the rescan position of the scanning carriages has been the most successful. Typically these dashpots take the form of cylinder and piston arrangements which, while in the extended position, are impacted by the scanning carriage of the optical system in its rescan path shortly before the carriage comes to rest at the start of scan position. This relatively high impact on the scanning carriage while better than no deceleration device, continues to risk the functional integrity of the lamps and mirrors. Under certain conditions it also may increase the operational wear and tear on all the bearing surfaces of the scanning carriage and any rails or other device that the carriage may ride on. It is believed that this is so because the deceleration profile is very abrupt, relying principally on impact of the scanning carriage with the deceleration device which is effective only over a very short distance.
This difficulty may be compounded in automatic copying devices having the capability of reproducing large or oversized original documents in either the same or reduced size. In these copying machines the length of the scanning path may be increased from traditional letter size for example to accommodate the greater dimension of the document. In so doing the scanning carriage is moved further from the dashpot and on the rescan path by the time it contacts the dashpot it is usually traveling at an increased speed due to the increased accelerating force placed on it by the stretched spring. In this mode of operation the probability of damage to the lamps and mirrors is further increased.